Reflecting incandescent lamp



lvlarch- 8, 1938. J, C Q JR 2,110,590

REFLECTING INCANDESCENT LAMP Filed Sept. 11, 1936 I? QM. b 7(mgk/1 W viorm gs Patented Mar. 8, 1938 REFLECTING INCANDESCENT LAMP Joseph Francis Cook, Jr., West Roxbury, Mam,

assignor to Birdseye Electric Corporation,

Dover, DeL, a corporation 01 Delaware Application September 11, 1936, Serial No. 100,285

7 Claims.

This invention relates to' improvements in the design and construction of incandescent lamps and particularly incandescent lamps having a part of the bulb surface coated with a reflecting material arranged to reflect a large part of the rays from the incandescent source and concentrate and direct them on the area of useful work. The particular improvements of this invention concern the shape and curvature of that part of the lamp bulb which determines the shape of the reflecting surface. I have discovered that improved results in certain fields may be secured by shaping a reflecting portion of-the bulb as a paraboloid of revolution, expanded until its focus, or the locus of its focal points, constitutes a circle of appreciable diameter and employing in the bulb an extended filament concentrically or symmetrically disposed with respect to such circle. An incandescent lamp so designed and constructed is efiective to throw a very bright spot of light on the center of the area to be illuminated, and a gradually diminishing amount of light on the surrounding area. That is, the area around the spot is brightly illuminatedless brightly than the spot however-and the intensity in this larger area of reduced illumination gradually fades out at the far edge of the illuminated area. Such lamps are useful in large store windows, for example, where the central exhibits require bright spots of light, and where the rest of the window requires .a reduced intensity of illumination, or in lighting gasoline filling stations, where much light is required near the pumps and a reduced amount elsewhere in the yard. The gradually reduced intensity of the outer zone permits these lamps to be grouped so that the light patterns overlap, without producing unsightly rings or bands of light and shade.

These and other features of the invention will be best understood and appreciated from the fo1- lowing description of preferred embodiments thereof, selected for purposes of illustration and shown in the accompanying drawing in which,-

Fig. 1 is a view of the lamp shown somewhat conventionally in cross section,

Fig. 2 is a similar view of a lamp of somewhat different design,

Fig. 3 is a polar diagram of the light emitted by the lamp of Fig. 2, and

Fig. 4 is a sectional plan view showing the location and shape of the V-shaped filament.

One satisfactory reflecting medium for lamps of my invention is metallic silver deposited upon the inner surface of the bulb in accordance with 5 the process of Pincus Deren disclosed in application Ser. No. 42,227, but the invention is in no sense limited to the employment of metallic silver nor to locating the reflecting medium upon the inner surface of the lamp bulb. For purposes of illustration, however, two lamps are shown which are provided with an inner coating of silver forming reflecting areas.

The bulb of the lamp herein shown is symmetrical about its axis and the reflecting surface is therefore also symmetrical, and may be accurately described as a surface of revolution, formed by rotating the trace l0l2 in Fig. l, for example, about the axis of the bulb. The trace of this reflecting surface, however, was not devel- 0 oped about the axis of the lamp. It bears a definite relation to this axis, and can be defined by a mathematical equation using the bulb axis as one of the rectangular co-ordinates, according to the practice of analytical geometry. How- 25 ever, it is much simpler to define the curvature of the reflecting surface in terms of the method by which it was designed. The trace of the reflecting surface from l@--l2 in Fig. 1, forexample, is a portion of a true parabola; however the 30 reflecting surface is not a paraboloid of revolu-v tion about the axis 20-22 of the parabolaas are the ordinary parabolic reflectors, such as those used in automobile headlights, for example. Such parabolic reflectors are designed to operate with a source of light thatis as nearly as possible a point source, and which is located in the main axis of the paraboloid of revolution and at its focus. In the production of commercial lamps of high wattage-from 100 watts and aboveit is impossible to make use of filaments that approximate a point source, since even pure tungsten would melt under such conditions. Generally a fine coil of fine tungsten or other'suitable wire is suspended in loops or in a circle of about in diameter to several times this size in lamps of high wattage. If such a filament is suspended at or near the focus of a true parabolic reflector, generated by revolving a half parabola about its axis, there is obtained not a spot-light but a mottled arrangement of brilliant and less brilliant light directly in front of the parabolic reflector with much "scattered and mottled light over a considerable area.

In the lamp of my invention I desired a strong spot of light directly in front of the reflecting bulb-in line with the axis of the lamp-surrounded by a field of diminishing intensity. However, no part of the field should be mottled with light of varying intensity, and for practical reasons a point source can not be used.

To accomplish this desired result I have designed the outer or lower part of the reflecting surface of my new lamp so that it is generated by revolving the parabola l0-|2, Fig. 1, about the axis of the mount, and therefore the axis of the bulb itself, but while maintaining the focus of the parabola in the circle of the fllament, the axis of the parabola being maintained parallel to the axis of the mount. Thus the shape of the parabolic portion of the reflecting surface becomes a paraboloid of revolution in which each element has been displaced outwardly with reference to the axis of the mount a distance equal to the radius of the circle of the filament. It will be seen that under these circumstances the locus of the focal points of all the displaced elements of this paraboloid of revolution lie in the circle which also contains the filament: or that the mraboloid has been expanded until its focal point becomes a locus of points which is a ring coinciding with the general contour of the filament or circumscribing the filament.

It will be understood that by a circular type filament I mean one which lies in a plane and which is approximately a circle or inscribed in or circumscribed about a circle so that the distance between the wire of the filament and the nearest point in the bulb is as uniform as possible. Such a circumscribing circle is shown in dot and dash lines in Fig. 4 and this may also represent the focal circle of the parabolic reflecting portion of the bulb. Obviously there must be an open space where the end of the circular filament joins the lead-in wires, and which cannot be closed without short circuiting the filament. A V-shaped filament, or a filament of almost any other shapeof which all parts are located in a single plane perpendicular to the axis of the mount, and disposed within the limits of the circle which is the locus of the focal points of the paraboloid and which are substantially uniformlyspread over the area of this circle, will produce satisfactory and substantially accurate results with the reflecting surfaces disclosed, since, as has already been stated the purpose of the lamp of this invention is to produce a bright spot surrounded by a field of gradually diminishing intensity of illumination rather than to produce a clear-cut light spot.

The coiled filament has two traces I4 and I 6 where it cuts the plane of the paper, as shown in the drawing Fig. 1. The filament trace I4 is at the focus ofthe partial parabola lfl-IZ, and the dotted curved line It indicates the completed half-parabola, having an axis 20-22 extending through the focus l4 and parallel to the main axis 2426 of the bulb. The other trace ii of the filament bears the same relation to the partial parabola on the near side of the drawing. Thus it will be noted that for each point on the filament there is a true parabolic reflecting surface, approximately at the focus of which lies that point of the filament. This true parabolic 9,1 1o,soo

surface with respect to each point on the filament however, is limited in extent in each case to a line lying in a plane determined by the point in question on the filament and the axis of the lamp. Those areas of the bulb-and therefore reflecting surface-lying adjacent to this line are approximately parabolic with respect to this same point on the filament and for an appreciable reflecting area surrounding this line approximately focus the light from this source. The rest of the reflecting surface, located above the plane of the filament reflects light falling on it from this same point source in a more and more widely divergent direction, divergence increasing with the distance from the parabolic axis, and this divergent light, from the infinite number of point sources which compose the circular filament comes an important part of the total used to illuminate the larger field of gradually diminishing intensity, surrounding the central brightly illuminated spot.

Thus it will be seen that the reflecting surface, with the parabolic trace, below the plane of the filament concentrates a considerable part of the light falling on it in a spot directly in front of the lamp and in a line with the axis of the bulb and tends to surround this spot with a field of gradually diminishing intensity.

Added to this effect is the direct light from the filament that passes through the transmitting portion 28 of the bulb, which, without any reflection, fans out with constantly diminishing intensity, on a plane surface perpendicular to the axis of the lamp, being brightest at a point nearest to and directly in front of the lamp, on the axis of the bulb, thus actually aiding in producing, the desired effect.

The rest of the reflecting surface within or above the plane of the filament requires special consideration. One of the great dimculties in using a true parabolic reflecting surface in connection with a filament operating under a relatively high wattage-400 watts for example-and with the filament located at the focus, is the fact that in parabolic reflectors of reasonable design, the focus is very close to the apex of the reflector. It is well understood that a parabola may vary in shape depending on the mathematical formula for the particular curve in question. It may be slender and narrow, or it may be broad and fiat. A narrow, slender parabola has its focus much closer to the apex of the parabola than a broad flat parabola, and the broad fiat type are not desirable for use in reflecting lamps of this type for several reasons, although it would be possible to use a broad fiat parabolic reflector, the focus of which was at a sufficient distance from the apex of the paraboloid so that an incandescent filament of relatively high wattage could be used without injuring the glass. Such broad flat parabolas, used as the basis of a bulb for a reflecting incandescent lamp, would make much too large and bulky a bulb for practical purposes-much larger than the normal volume requirements for a given wattage, and this effect would be greatly exaggerated if the parabolic surface were extended soas to hood the filament, and produce a cone of light, the outside edge of which would make some desirable angle with the axis of the cone, of say, 60 or 75. In the past, efforts to avoid this diificulty with a parabolic reflecting lamp have resulted in an arbitrary enlargement of the lamp bulb at the apex of the parabolic cone, simply to make greater the distance, between the bulb and the a waythat the'reflected rays from this part of the reflecting surface act as much as possible in substantially the same wayv as those from the parabolic part, and actually serve to reinforce the eflect of the reflected light from the parabolic surface. Moreover, by this design, although from neither part of the reflecting surface is all of the reflected light sent out in parallel rays, a large part of the reflectedlight is so emitted, thus forming the desired spot at the center of the illuminated area, and all the rest of the light from the filament is emitted in a larger surrounding cone of gradually diminishing intensity, brightest at the center and diminishing to a low valueat the edge of the cone. As has been pointed out above,

this gradual diminution of intensity toward the edge of the illuminated field, without mottling, is a most important characteristic of this particular lamp. The direct light from the filament, as is well understood, meets this requirement; that from the parabolic part of the reflector does, also, as has been explained above; and the reflected light from the globular part of the reflecting surface also does, since it is used to reinforce the reflected light from the parabolic part of the reflector, as is shown directly below.

The globular part of the reflecting surface is composed partly of the reflecting surface of the bulb, indicated by the trace l0--30 in Fig. 1, and partly of the reflecting barrier, 32, which is preferably a bright white metal upwardly convex disk supported in any desired manner in the location suggested in Figs. 1 and 2. The curve of this combined reflecting surface is continuous, except for a small space between the bulb walls and the reflecting barrier 32, and the curve is a somewhat flattened circle and may be considered to be an ellipse of which two points M and I6 are the two foci, these points also being the trace of the circular type filament with the plane of the paper. It is well understood that a reflector shaped in the curve of an ellipse and receiving an incident ray from a source located at one focus will reflect the my back to the other focus. In any case the curve as shown and as used for this symmetrical globular portion of the reflecting surface is such that light striking. it in the plane of the paper, for example, from trace l6 of the filament will be reflected back through trace l4 and vice versa. Thus such a ray, after being reflected by the globular portion of the, reflecting surface will appear to emerge from a point on the filament directly opposite the point on the filament where it was originally generated. It will thus act merely to reinforce the light coming from the point on the filament directly opposite and will either appear to emerge through the transmitting portion of the bulb as a direct ray or be reflected a second time from the parabolic part of the reflecting surface, and, for example, emerge as one of the parallel rays forming the centrally located bright spot.

In the elliptical reflectorof my invention, it is apparent that only the rays leaving the filamenttrace l6 in the planeof the paper will be accurately reflected through the opposite filament trace I 4. All other rays leaving trace ii on the filament will be reflected by the globular part of the reflecting surface through points near the trace l4, and at gradually increasing distances from II as the angle of the plane containing the ray under consideration makes a greater and greater angle with the plane of the paper. On all other planes through the axis of the lamp. there will be two traces similar to I and I6, and to light from this infinite number of point sources on the circular filament can be applied this same dis cussion. Thus it will be seen, that light reflected from the globular part of the reflecting surface,

is in part accurately emitted as parallel rays to 15 produce a centrally located brilliant spot of light, and the rest of the light will be distributed over a larger area, and at an intensity which gradually diminishes as the distance from the central spot of light increases. This gradually diminishing intensity feature of the larger area around the central spot is of great importance, and it is apparent that any abrupt changes in reflecting angles of the reflecting rays, from any point source on the filament under consideration, as they gradually diverge from the direction that produces parallel rays, would tend to produce an area of rapidly changing intensity, and a mottled illuminated area would result.

' Thus it is apparent that the reflected light from the globular reflecting surface tends to reinforce the effect from the parabolic portion, and that light from these two surfaces together with the direct light from the filament all contribute to the desired final result of a bright spot, centrally located, surrounded by a larger area of gradually diminishing intensity, and without any abrupt changes in the intensity of illumination of the floor area, for example, below the lamp.

The globular portion of the bulb has also the important function of spacing the reflecting coating sufllciently from the incandescent filament to permit convection currents of gas to pass across the reflecting surface and by the distance and cooling effect thus provided to obviate damage to the coating which might otherwise be caused by over-heating.

I do not desire to be limited in my invention to the exact design, shape and measurements shown in the accompanying drawing, since it is apparent that larger sizes, for example, or lamps giving a'larger bright spot, or a less marked difference in illumination between the bright spot and the surrounding less brilliant area, could readily be designed and manufactured from the disclosure given and without departing from the actual invention.

The lamp shown in Fig. 2 is of slightly different shape from that illustrated in Fig. 1 but it corresponds in construction thereto. The portion generated by revolving parabolic trace 50-52 about the axis 64-66 of the bulb is provided with an internal coating 5| of metallic silver. The parabolic axis of this trace is shown at 60-62. The small circles 54-56 indicate the traces with the plane of the paper of a ring shaped filament which is the locus of the focal points of all the parabolic elements of this surface. The portion of the bulb generated by revolving the trace 50-10 about the axis of the bulb, together with the upwardly convex reflecting barrier 12 constitute a flattened spherical reflecting surface acting upon the light rays as already explained in connection with the lamp of Fig. 1. The flattened transmitting portion Bl may be clear or frosted as desired and, of course, may have a greater or lesser radius of curvature than as illustrated. The stem, mount and lead-in wires are omitted in Figs. 1 and 2 since they may be of any conventional or commercial construction.

As one example of a bulb of satisfactory shape to accomplish the results above discussed, the

- dimension of that shown in Fig. 2 may be as fol- In Fig. 3 is shown a polar diagram showing the distribution and intensity of the light emitted by the lamp of Fig. 2. This indicates a very high intensity of light within a cone having an included angle of 26, a concentric cone of rapidly decreasing intensity out to an included angle of 76 and an outer concentric cone of less rapidly decreasing intensity out to an included angle of 100. It will be understood that the light distribution indicated in this diagram is merely typ= ical of that derived from one lamp constructed in accordance with my invention, and that the angles mentioned may be varied within a considerable range by appropriate modification of the bulb dimensions.

Having thus described my invention, what I claim as new and desire to secure by Letters Patentof the United States is,

1. A reflecting incandescent lamp having a bulb shaped and coated to present a reflecting surface which is a paraboloid of revolution expanded until the locus of its focal points constitutes a circle .of appreciable diameter, a filament located in the plane of the focal points of said parabola, and a concave globular reflecting surface of different curvature located behind the focal plane of the parabolic portion of the bulb and shaped so as to aid in reflecting rays from the lamp in a'cone of divergent rays of intensity diminishing from the center outwardly.

2. A reflecting incandescent lamp having a bulb shaped and coated to present a reflecting surface which is a paraboloid of revolution expanded until the locus of its focal points constitutes a circle of appreciable diameter, a filament located in the plane of the focal points of said parabola and acting to supply a reinforced beam of light as substantially parallel rays, and a flattened hemispheric-a1 reflecting surface located behind the focal plane of the parabolic portion of the bulb and shaped to reflect rays from the filament so that these reflected rays reinforce the partial focusing action of said parabolic portion of the reflecting surface.

3. A reflecting incandescent lamp having a bulb with a mount therein, and a filament coiled in helical fashion and arranged as a flat loop, a portion of the surface of the bulb serving as an eflicient reflecting surface and being divided into two parts, one a paraboloid of revolution which is a surface generated by revolving a partial parabola about the axis of the bulb and at a constant distance from said axis, so that the focus of the parabola transcribes a circle about said axis, and said circle determining the position of said filament, and a second reflecting portion beyond the first, the surface of which is a flattened hemisphere and shaped to reflect the light from one or more points on the filament back to the filament at a point directly across from the point of emission, said second reflecting portion thereby shaped to reinforce the light falling on the parabolic reflecting surface.

4. A reflecting incandescent lamp having a symmetrical bulb, a mount and a filament arranged concentrically with respect to the bulb, the bulb presenting cooperating reflecting surfaces of which the principal surface is formed by revolving a parabola about the axis of the mount at a constant distance therefrom so that the focus of the parabola describes a circle about the axis of the mount and this circle coincides substantially with the circle bounding the filament, the said parabolic surface having its focus relatively close to its apex and terminating at the level of the plane containing the filament, the secondary portion of the reflecting surface being globular in shape and located between the filament and the base of the lamp.

5. A reflecting incandescent lamp including in its structure a bulb having a filament arranged in a plane at right angles to the main axis of the bulb, the bulb being shaped and coated to present an integral and continuous reflecting surface including a substantially parabolic portion truncated and terminated approximately at its focal plane and supplemented by an enlarged globular portion located between the filament and the base of the lamp, the plane of said filament coinciding substantially with the focal plane of the parabolic portion of the bulb and both portions of the bulb cooperating to direct the light of the lamp in a concentrated beam, while the'globular portion furnishes space for convection currents of gas tending to cool the reflecting coating of the bulb and so prevent damage from overheating.

6. A reflecting incandescent lamp including in its structure a symmetrical bulb having a parabolic outer portion and a bulging elliptical inner portion, a mount, and a filament disposed in a single plane which is perpendicular to the axis of the mount and which includes the focus of said parabolic portion, the bulb providing a reflecting surface formed by revolving about the axis of the mount, and at a distance therefrom, a compound continuous curve, the outer part of which is a partial parabola and the inner part of which is a partial ellipse, the two parts of such curve having a common focal point which lies in the said planeof the filament of the lamp and describes a circle in the revolution of said compound curve.

7. A reflecting incandescent lamp including in its structure a bulb having a coiled filament arranged in a plane at right angles to the main axis of the bulb, the bulb being shaped and coated to present an integral and continuous reflecting surface including a substantially parabolic portion truncated and terminated approximately at its focal plane and supplemented by an enlarged globular portion located between the filament plane of the parabolic-portion of the bulb and both portions of the bulb cooperating-to direct the light of the lamp in a concentrated beam,

while the globular portion furnishes space for convection currents of gas tending to cool the reflecting coating of the bulb and so prevent damage from overheating, and a reflecting disk located in the bulb as a substantial continuation of said globular portion.

JOSEPH FRANCIS COOKJIRY. 

